The popular name for the glass used in the making of colored windows, which really should be confined to glass painted with oxide of silver. Pot-metal, the glass usually employed in colored windows, is colored while in a molten state. Before the Christian era colored windows were made in the East by arranging small pieces of pot-metal. This process was adopted by the early Gothic builders and the pieces of glass were secured with strips of lead. The first painted windows were not durable but the art was revolutionized when the process of painting with metallic pigments fused into the glass was discovered. The new art had its center at Chartres. The method of painting prevailed over that of mosaic. At first figures were on medallions imbedded in a border. The 13th century produced the jewel-like glass arranged in harmonious and beautiful compositions. In the 14th the yellow stain was discovered and the grisaille window of sober colors was introduced. In the 15th and 16th centuries glass was treated as canvas and the art degenerated. Later the use of pot-metal was almost abandoned for paint and enamels. The 19th century revival was most successful in England where medieval methods were studied. The opal glass used in America gives beauty of color but lacks the effect of glass used in older countries.